Recently, in the radio communication field, high-speed packet communications such as HSDPA (High Speed Downlink Packet Access) have been attracting attention. In HSDPA, packets are transmitted at the optimal transmission rate according to the downlink propagation environment, and therefore an adaptive modulation scheme is used for packet transmission from a base station apparatus.
In a radio communication system in which an adaptive modulation scheme is used, a mobile station apparatus obtains a CQI (Channel Quality Indicator), which is indicative of propagation environment, from the reception quality of a receive packet, and reports this CQI to the base station apparatus that transmitted the packet. Then, in order to transmit packets at the optimal transmission rate according to the reported CQI, the base station apparatus selects a transmission scheme that will achieve that transmission rate. A transmission scheme that will achieve that transmission rate is decided upon based on the coding rate, number of codes multiplexed, modulation scheme and so forth, for example. By this means, a desired PER (Packet Error Rate), neither excessive nor inadequate, can be achieved by a mobile station apparatus.
Since a base station apparatus selects an optimal transmission rate according to the reported CQI in that way, using an adaptive modulation scheme is based on the important premise of accurately reported CQIs. Consequently, methods of testing the accuracy of a reported CQI have heretofore been proposed.
For example, 3GPP, R4-021533 “VRC Test Approach”, TSG-RAN Working Group 4 (Radio) meeting #25 Secaucus, N.J., USA, 11th-15th Nov. 2002 described a method whereby packets are continuously transmitted for a fixed period using a modulation scheme, coding rate, and transport block size corresponding to a fixed CQI, and the PER and throughput in that case are measured.
Specifically, packets are transmitted for a fixed period to a communication apparatus under test such as a mobile station apparatus, with a transmission rate (that is, a modulation scheme, coding rate, and transport block size) corresponding to a fixed CQI, without regard to the reported CQI from that communication apparatus. During this period, the communication apparatus performs CQI reporting and also reports Ack/Nack indicating whether or not a transmitted packet has been received correctly. Then, using the reported CQI and Ack/Nack for the same packet, the PER is calculated for each reported CQI value, and results such as those shown in FIG. 1 are obtained.
In FIG. 1, when the CQI is fixed at 10, for example, (the middle graph in the figure), if the reported CQI, which should be 10, is less than 10, this indicates that the propagation environment is poor—in other words, that the communication apparatus cannot correctly receive a packet transmitted at a transmission rate corresponding to the CQI of 10. Therefore, when the reported CQI is less than 10, the PER is higher.
On the other hand, if the reported CQI is 10 or above, this indicates that the propagation environment is good—in other words, that the communication apparatus can correctly receive a packet transmitted at a transmission rate corresponding to the CQI of 10. Therefore, when the reported CQI is 10 or above, the PER is low.
By using such test results, it is possible to decide upon criteria such as the PER upper limit and throughput lower limit for each reported CQI value, and to determine whether or not the criteria are met for each reported CQI.
However, a problem with a method whereby packets are continuously transmitted for a fixed period at a transmission rate corresponding to a fixed CQI and reported CQI accuracy is measured, as described above, is that the CQI is not fixed at a value that is optimal for all communication apparatuses. That is to say, a communication apparatus incorporating advanced receivers such as an equalizer and interference canceler, for example, has reception performance higher than a communication apparatus that does not have such advanced receivers, and therefore the propagation environment is determined to be better than it actually is, and a CQI higher than the fixed CQI is reported frequently as the reported CQI, which consequently results in invalidity of such a reported CQI accuracy measurement test.
Also, if a communication apparatus tends to report a CQI lower than is actually the case as a reported CQI, the PERs for reported CQI values will become lower on the whole. There is thus a problem of PERs corresponding to reported CQI values easily falling below the respective decided PER upper limits, resulting in erroneous passing of the test. If such a communication apparatus that lowers a reported CQI is present in a radio communication system, packets will be transmitted at a low transmission rate overall, and overall system throughput will fall.